Cutting tools for chip forming metal machining consist of a substrate body of cemented carbide, cermet, ceramics, steel or cubic boron nitride that is usually coated with a single-layered or multi-layered hard coating to improve cutting properties and wear resistance. The hard coating consists of polycrystalline mono-metallic or multi-metallic hard phases. Examples of mono-metallic hard phases are TiN, TiC, TiCN and Al2O3. Examples of multi-metallic hard phases are TiAlN and TiAlCN. The hard phase coating layers are deposited on the substrate by CVD or PVD methods.
Polycrystalline hard phase coatings deposited by CVD or PVD methods can be grown with strong preferential crystallographic orientation, also called fiber texture. In recent developments to improve cutting performance and wear resistance of coated cutting tools, CVD and PVD coatings have been grown with different preferential crystallographic orientations, i. e. fiber textures, each of which may yield favorable performance in different cutting operations due to the anisotropic properties of the coating materials. An example is the use of highly {0 0 1} textured α-Al2O3 coatings in turning operations with the crystallographic planes {0 0 1} preferentially oriented parallel to the substrate surface.
U.S. Pat. No. 7,767,320 discloses a hard coating deposited by CVD and comprising a layer of face-centered cubic (fcc) Ti1-xAlxN with 0.75<x<0.93 and a process for its production.
U.S. Pat. No. 8,257,841 discloses a hard coating deposited by CVD comprising a layer of TiN, TiCN or TiC deposited directly on the substrate surface, followed by an adhesion layer having a phase gradient and a subsequent layer of TiAlN. The TiAlN layer has a fiber texture with the crystallographic {2 0 0} planes preferentially oriented parallel to the substrate surface.
WO 2009/112116 discloses hard coatings of TiAlN, TiAlC or TiAlCN having a high content of Al and a face-centered cubic (fcc) crystal lattice and being deposited by CVD on top of a TiCN or Al2O3 layer. It is not disclosed whether the coatings have preferential crystallographic orientation. WO 2009/112117 discloses hard coatings comprising a layer of (Ti, Me)Al(C,N) with Me=Zr or Hf to increase the hardness of the layer. WO2009112115A1 teaches a body with a hard coating comprising an outer Al2O3 layer on top of a TiAlN, TiAlC or TiAlCN layer.
The most commonly used CVD coatings to increase the wear resistance of cutting tools are α-Al2O3 coatings and TiCN coatings deposited by moderate temperature CVD (MT-CVD).
Bartsch et al. have obtained TiC coatings with the crystallographic planes {1 1 1} preferentially oriented parallel to the substrate surface by the use of aromatic hydrocarbons as precursors at deposition temperatures ≥1000° C. These coatings provided superior wear resistance compared to TiC coatings with the crystallographic planes {1 0 0} preferentially oriented parallel to the substrate surface in turning of cast iron (K. Bartsch et al., Advances in Inorganic Films and Coatings (1995), 11-18).
For TiCN coatings it is known that good wear resistance, especially resistance against flank wear, can be achieved with coatings produced by the moderate temperature CVD (MT-CVD) process, as opposed to high temperature CVD (HT-CVD) processes. The MT-CVD process is run in a temperature range of 675-950° C., and makes use of nitrile compounds, most commonly acetonitrile, to yield so called MT-TiCN coatings with a columnar microstructure which is considered favorable for metal cutting. MT-TiCN coatings have been reported to have different crystallographic fiber textures.
Larsson and Ruppi (Thin Solid Films 402 (2002) 203-210) compare the metal cutting properties of untextured TiCN coatings deposited by high temperature CVD (HT-CVD) exhibiting a microstructure with equiaxed grains, and {211} textured MT-TiCN coatings with columnar structure. The MT-TiCN coating has a better chipping resistance, but lower resistance to crater wear than the HT-TiCN coating.
U.S. Pat. No. 6,756,111 discloses a multi-layer coating with an outer MT-TiCN layer having any one of {110}, {311}, {331} or {211} fiber texture.
U.S. Pat. No. 8,012,535 discloses MT-TiCN coatings obtained in the temperature range of 880-970° C. with the addition of a monocyclic hydrocarbon, e.g. benzene, to the gas phase yielding coatings with {221}, {331} or {110} fiber texture.
U.S. Pat. No. 7,348,051 discloses MT-TiCN coatings with a preferential crystallographic orientation with the crystallographic planes {1 1 2} preferentially oriented parallel to the substrate surface or with a deviation of less than 10 degrees from that, as determined by EBSD.
EP 2 604 720 A1 discloses a tool with a columnar fine grained MT-TiCN coating layer having an average grain width of 0.05 μm to 0.4 μm and a carbon content (C/(C+N)) of 0.50 to 0.65. The columnar MT-TiCN layer has a strong {211} fiber texture with considerable {311} fiber texture component, and it comprises twinned columnar grains.
Twin formation in TiCN CVD coatings is a well-known phenomenon. Using the coincidence site lattice (CSL) formalism, a twin can be described as Σ3 grain boundary, and high twin formation in TiCN CVD coatings correlates with a high relative length of Σ3 grain boundaries of the sum of grain boundaries of ΣN-type
EP 1 626 105 A1 discloses a TiCN layer of thickness between 3 μm and 20 μm having a high relative length of Σ3 grain boundaries as defined by the ratio of lattice points of Σ3 to ΣN with N=2n+1, 1≤n≤14 in the range of 60% to 80%, in contrast to ≤30% Σ3 grain boundaries found in conventional coatings.
EP 1 897 970 discloses a coated cutting tool comprising a columnar TiCN layer with a carbon content (C/(C+N)) of 0.7 to 0.9, and with an average grain size parallel to the surface (grain width) of 0.05 μm to 0.5 μm. The XRD peak ascribed to the {4 2 2} crystallographic plane has a half width of 0.40° to 0.60°, and is preferably the peak having the highest intensity.
WO 2012/126030 discloses a body with a multi-layer coating comprising an AlxTi1-xN layer deposited on a TiCN layer with elongate crystals. The majority of the AlxTi1-xN layer has the cubic crystal structure, however, it comprises up to 30 mole-% of hexagonal phase AlN. The coating system is described to show enhanced wear resistance compared to PVD AlxTi1-xN coatings.
Generally, for milling of cast iron and steel the use of CVD coated cemented carbide grades is preferred over PVD coated grades, especially for applications using high cutting speeds.
Typical CVD coated milling tools have a multi-layer coating comprising a thin TiN adhesion layer immediately on the substrate surface, an inner MT-TiCN layer and an outermost α-Al2O3 layer as the main wear resistant layer. While CVD coated cutting tools offer good wear resistance at high cutting speeds, the resistance to thermo-mechanical shocks occurring in intermittent cutting is still limited. Therefore a typical wear mechanism of milling tools is the occurrence of thermal cracks or comb cracks, respectively, on the primary cutting edge.